ST. LOUIS (AP) — Guests can get paid $3,500 to stay at a St. Louis hotel for up to 12 days with catered meals, as long as they sign up to be exposed to the flu virus.

A St. Louis University research unit is testing the effectiveness of flu vaccines by paying volunteers to stay at its Salus Center, formerly the Water Tower Inn, the St. Louis Post-Dispatch reported . Participants will be given a flu shot or a placebo, and then receive a dose of a flu virus through nasal spray.

The volunteers will then be observed for flu symptoms, such as fever, runny nose, sneezing or coughing. The Extended Stay Research Unit will repeatedly test participants’ blood and mucus for signs of the virus.

The university spent about $350,000 to convert 24 hotel rooms at the Salus Center into a quarantined medical unit, equipped with reading nooks and game tables for socializing.

The “human challenge” unit is one of only a few across the world that’s designed to test vaccines or treatments on people after exposing them to a disease.

Dr. Daniel Hoft, director of the university’s Center for Vaccine Development, said “you can learn a lot more, a lot faster” about whether vaccines work to prevent infection by controlling the study environment.

The 2017-18 flu season was one of the most severe on record. About 134,000 infections and 279 deaths were reported in Missouri. The seasonal flu shot was between 10 and 15 percent effective against the strains, Hoft said.

“The real question for me ethically is, if you’re not benefiting people, you have to be careful about not harming them too much, and they know what they are getting into and are freely choosing it,” said Stephanie Solomon Cargill, an associate professor of health care ethics at the university. “You wouldn’t want to expose someone to a permanent illness with no cure.”